The Kepler mission [1]} demonstrated that planets with sizes between that of Earth and Neptune, with no counterpart in our Solar System, orbit about half of all planetary systems with Sun-like hosts [2]}, [3]}. The origin and nature of these planets are still under debate [4]}, but the bimodality of their size distribution — with a dearth of planets with radii between 1.7 and 2.0 \(R_\oplus \)  [5]}, [6]} for solar-type stars and between 1.5 and 1.7 \(R_\oplus \)  [7]}, [8]}, [9]} for M-dwarf stars — indicates that their composition ranges between scaled-up versions of Earth and the scaled-down versions of Neptune. These planets could be made of rock, ices, and/or gas, since multiple combinations of these materials can match the observed mean densities [10]}. Only by combining precise bulk density measurements with elemental stellar abundances is it possible to break the degeneracies of the internal composition models to derive reliable rock, water, and atmospheric mass fractions [11]}, [12]}.
